1. Field of the Invention
The present invention relates to a hydrogen filling apparatus and a hydrogen filling method for filling a hydrogen tank with hydrogen to be supplied to a fuel cell or the like.
2. Description of the Related Art
Fuel cells for generating electrical energy by way of an electrochemical reaction between hydrogen and oxygen are supplied with hydrogen (hydrogen gas) as a fuel from a hydrogen tank. The hydrogen tank stores hydrogen under a predetermined high pressure and is located near the fuel cell. The hydrogen tank is usually filled with hydrogen supplied from a hydrogen storage tank which stores hydrogen under a higher pressure.
Japanese Laid-Open Patent Publication No. 2007-309375 discloses an apparatus capable of filling a tank with a high-pressure gas such as a hydrogen gas while preventing a temperature rise in the tank. Though the disclosed apparatus is advantageous in that it can reduce the time required to fill the tank with the high-pressure gas because the temperature of the tank is prevented from increasing, the operator finds it hard to recognize the time needed to complete the filling process.
Japanese Laid-Open Patent Publication No. 2007-138973 also discloses a method of measuring, per given time, a change in a physical quantity such as the flow rate or pressure of filling hydrogen when a hydrogen storage alloy container is filled with hydrogen, approximating the measured change with an approximate expression, and calculating a predicted time until the completion of the filling process according to the approximate expression.
According to the method disclosed in Japanese Laid-Open Patent Publication No. 2007-138973, the predicted time until the completion of the filling process is calculated based on a given physical quantity such as the flow rate or pressure of filling hydrogen in the filling process. As the physical quantity tends to fluctuate, the accuracy with which to calculate the predicted time until the completion of the filling process is likely to decrease. Furthermore, since the filling process is continuously carried out at all times when the physical quantity is measured, if hydrogen tanks to be filled have different volumes, the accuracy with which to calculate the predicted time until the completion of the filling process becomes lower. Consequently, there has been demand for a hydrogen filling apparatus which enables an operator to recognize easily a more accurate time needed to complete the filling process for further improving its convenience for the operator.